hey there!
Let measure of each angle be x
In triangle , we know that sum of angles is 180°
In equilateral triangle , all angles are equal
So our angles are x , x , x
=> x + x + x = 180°
=>3x= 180°
=> x = 180/3
=> x = 60°
Option 3 is correct
Hope this explanation helps you :)
Have a good day :)